On Oct 14, 2016 9:14 AM, "Gustavo Carneiro" <gjcarneiro@gmail.com> wrote:
>
> Sorry if I missed the boat, but only just now saw this PEP.
>
> Glancing through the PEP, I don't see mentioned anywhere the SQL alternative of having a coalesce() function: https://www.postgresql.org/docs/9.6/static/functions-conditional.html#FUNCTIONS-COALESCE-NVL-IFNULL
>
> In Python, something like this:
>
> def coalesce(*args):
> for arg in args:
> if arg is not None:
> return arg
> return None
>
> Just drop it into builtins, and voila. No need for lengthy discussions about which operator to use because IMHO it needs no operator.
>
> Sure, it's not as sexy as a fancy new operator, nor as headline grabbing, but it is pretty useful.
That function is for a different purpose. It selects the first non-null value from a flat collection. The coalesce operators are for traveling down a reference tree, and shortcutting out without an exception if the path ends.
For example:
return x?.a?.b?.c
instead of:
if x is None: return None
if x.a is None: return None
if x.a.b is None: return None
return x.a.b.c
You can use try-catch, but you might catch an irrelevant exception.
try:
return x.a.b.c
except AttributeError:
return None
If `x` is an int, `x.a` will throw an AttributeError even though `x` is not None.
A function for the above case is:
def coalesce(obj, *names):
for name in names:
if obj is None:
return None
obj = getattr(obj, name)
return obj
return coalesce(x, 'a', 'b', 'c')
See this section for some examples:
https://www.python.org/dev/peps/pep-0505/#behavior-in-other-languages
(The PEP might need more simple examples. The Motivating Examples are full chunks of code from real libraries, so they're full of distractions.)